Why do we not have sand boxes on all the tees so people can repair divots?

The sand boxes are surprisingly expensive (due to them only being made by a small number of companies). In order to keep costs down the sand boxes are only placed on tees which get the most divots. e.g. par 3s and shorter par 4s where irons are used to tee off.
It should also be noted that the existing boxes do not get fully used by golfers. In addition to lots of unrepaired divots being left on the tee, we have to throw away a lot of the grass/sand mix as it cannot be used effectively once it has started to germinate.